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BaSn3 A Superconductor at the Border of Zintl Phases and Intermetallic Compounds. Real-Space Analysis of Band Structures

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[18] G. W. Bushnell, G. V. Lourie, G. D. Brayer,1 Mol. Biol. 1990,214,585-595.
[19] NMR analysis shows that 2 takes up an intramolecularly hydrogen-bonded
conformation similar to that seen in the cyclo-(Ala-Arg-Gly-Asp-Mamb)
3: To a solution of 1 [14,15] (86 mg, 0.10 mmol) and oxalyl chloride (254 mg,
loop [17b].
2.0 mmol) in dry CH,Cl, (10 mL) was added DMF (0.025 mL) through a silica gel
I201 Cytochrome c linked to agarose was purchased from Sigma and packed in a
filter, and the mixture was stirred at room temperature for 8 h. The reaction
5 x 125 mm column. A mixture of 1.2mmol of 3 and 2.5 mmol of 4 in Tris
mixture was evaporated in vacuo to obtain the acid chloride (102 mg). A solution
buffer (10mM, pH=7.4) was applied and eluted with the same buffer
of 2 [17] (266 mg, 0.44 mmol) and diisopropylethylamine (DIEA, 80 mg,
solution with stepwise increase in NaCl concentrations (20 mL each of 0.0.5,
0.60 mmol) in dry CH2C12(10 mL) was added to the evaporated residue. The
2, and 4 ~ ) .
mixture was stirred at room temperature for 14 h. The reaction mixture was
[21] T. Ozawa, M. Tanaka, T. Wakabayashi, Proc. Nutl. Acad. Sci. USA 1982,79,
purified by preparative thin-layer chromatography (SiO,; eluted with MeOW
7175-7179.
CH2C12,first 119 and later 114) to afford the octa-ten-butyl ester derivative of 3 as
[22] G. Mclendon in Control of Eiologicul Electron Transport via Molecular
a yellow powder (281 mg, 89 YO):m.p. z 350°C; ’H NMR (300 MHz, [D,]DMSO):
Recognition and Binding: The “Velcro” Model; (Eds.: M. J. Clark, J. B.
6=10.10(br.s4H),8.95(br.,4H),8.49(br.,4H),8.25(d,J=8.7Hz,4H),8.09
Goodenough, J. A. Ibers, C. K. Jorgensen, D. M. P. Mingos J. B. Neilands,
(br., 4H), 7.99 (m,8H). 7.67 (s, 4H), 7.60 (s, 8H),7.37 (s,4H),4.81 (m,8H), 4.51
G. A. Palmer, D. Reinen, P. J. Sadler, R. Weiss, R. J. P. Willlams), Springer,
(m,8H),3.99(m,20H),3.63(m,8H),3.46(m,4H),2.79(m,4H),2.62(m,4H),
Heidelberg, 1991, p 165.
2.31 (m, SH), 1.97 (m.8H), 1.47 (m, SH), 1.35 (m, 72H), 1.02 (t, J=8.4Hz,
[23j CrystaI structure of the compIex between cytochrome c and cytochrome c
12H); I3C NMR (75 MHz, [D,]DMSO): d = 120.1, 117.8, 80.0 (2C), 75.0, 49.1,
peroxidase: H. Pelletier, J. Kraut, J. Science 1992,258, 1748- 1755.
48.8, 44.2, 42.3, 41.4. 37.5, 36.9, 31.9, 30.5, 27.6 (ZC), 18.9, 14.0.
[24] Calculated with the program MacroModel of W. C. Still, Columbia
The octaester (298 mg, 0.093 mmol) was added to “FA (3 mL) and dry CH2C12
University, New York (USA).
(3 mL), and the mixture was stirred at room temperature for 1 h. The mixture
[25] ‘H NMR titration conditions: [Cyt c] = 3 . 0 m ~in 12mM KNO,, pD = 9.3,
was evaporated under reduced pressure. The crude product was passed through
[3]=0-9mM.
anion exchange resin (Amberlite IRA-400(OH), water) and cation exchange
[26] NMR titration of cytochrome c with cytochrome c peroxidase: J. D.
resin (Amberlite IR 120 (plus), water) to remove ions. Water was lyophilized to
Satterlee, S. J. Moench, J. E. Erman, Biochim. Eiophys. Acta 1987, 912,
give 3 (229 mg, 90%): m.p. > 350°C; ‘H NMR (300 MHz, [D,jDMSO): d = 10.02
87 - 97.
(s,4H), 8.81 (s,4H),8.48 (s,4H), 8.31 (d,J=8.7 Hz,4H), 7.97 (m, 12H),7.64 (5
1271 E. Mochan, P.Nicholls, Eiochim. Biophys. Acta, 1972,267,309-319.
4H), 7.55 (s, 8H), 7.30 (s, 4H), 4.76 (m, 8H), 4.52 (m, SH), 4.0-3.4 (m, 32H).
[28] L. C. Petersen, R. P. Cox, Biochem. 1.1980,192,687-693.
2.8-2.6 (m, 8H). 2.5-2.3 (m, 8H), 1.97 (m. 8H), 1.50 (m,8H), 1.01 (t, 8H); ”C
NMR (75MHz, [D,]DMSO): 6=172.2, 171.7, 170.9, 170.4, 169.3, 168.5 (2C).
165.1. 159.0, 140.2. 138.8, 134.4, 134.2, 128.7, 128.3, 121.4, 119.9, 117.7, 75.0, 49.1
(2C). 44.3, 42.2, 41.3, 36.1 (2C). 31.9.30.5, 18.9, 14.0; high-resolution fast-atombombardment mass spectrometry (matrix: 3-nitrobenzyl alcohol): m1z calcd for
C128H14sN240,
(M+H+) 2721.9767; found 2721.985.
Experimental Section
Received: June 12,1997 [Z10543IE]
German version: Angew. Chem. 1997,109,2797- 2799
Keywords: calixarenes
cytochrome c
recognition peptidomimetics proteins
-
-
-
molecular
~
[l] For a general discussion of protein-protein recognition sites, see J. Janin, C.
Chothia, 1 Biol. Chem. 1990,265, 16027- 16030.
[2] C. Branden, J. Tooze, Introduction to Protein Structure, Garland, New York,
1991,p. 187.
[3] I. A. Wilson, R. L. Stanfield, J. M. Rini, J. H. Arevalo, U. Schulze-Gahmen,
D. H. Fremont, E. A. Stura, Ciba Found. Symp. 1991159.13-39.
[4] D. R. Davies, S. Chacko, Acc. Chem. Res. 1993.26.421 -427.
[5] a) C. Chothia, A.M. Lesk, A. Tramontano, M. Levitt, S. J. Smith-Gill, G.
Air, S. Sheriff, E. A. Padlan, D. Davies, W. R. Tulip, P. M. Colman, S.
Spinelle, k? M. Alzari, R. J. Pojak, Nature 1989,342,877-883; b) C. Chothia,
A. M. Lesk, J. Mol. Biol. 1987,196,901-917.
[6] See P. Hollinger. H. R. Hoogenboom, Trends Biotech. 1995,13,7-9.
[7] C. E Barbas, J. D. Bain, D. M. Hoekstra, R. A. Lerner, Proc. Natl. Acad. Sci.
USA 1992,89,4457-4461.
[8] J. Ku. P. G. Schultz. Proc. Nutl. Acad. Sci. USA 1995,92,6552.
[9] M. Mutter, P. Dumy, P. Garrouste, C. Lehmann, M. Mathieu, C. Peggion, S.
Peluso, A. Razaname, G. Tbcherer, Angew. Chem. 1996,108, 1588-1591;
Angew. Chem. Int. Ed. Engl. 1996,35,1482-1485.
[lo] J. 0. Magrans, A. R. Ortiz, M. Antonia Molins, P. H.P. Lebouille, J.
Sanchez-Quesada, P. Prados, M. Pons, F. Gaga, J. de Mendoza, Angew.
Chem. 1996,208,1816-1819; Angew. Chem. Int. Ed. Engl. 1996,35,17121715.
[ l l ] L. C. Hsieh-Wilson, X-D, Xiang, P. G. Schultz, Acc. Chem. Res. 1996, 29,
164- 170.
[12] a) C. D. Gutsche, M. Iqbal, D. Stewart, J. Org. Chem. 1986,51,742-745; b)
C. D. Gutsche, L-G. Lin, Tetrahedron 1986,42,1633-1640; c) M. Canner, V.
Janout, S. L. Regen, 1.Org. Chem. 1992,573744 - 3746.
[13] Calix[S]arene and calix[6]arene are also available and offer the potential for
extending this approach to analogues with 5- and 6-loops
[14] A related example: A. Casnati, M. Fabbi, N. Pelizzi, A. Pochini, F. Sansone,
R. Ungaro, E. Modugno, G. Tarzia, Bioorg. Med. Chem. Lett. 1996,6,26992704; A. Marra, A. Dondoni, F. Sansone.1 Org. Chem. 1996,61,5155 -5158.
1151 The syntheses of analogous compounds were described in R. H. Vreekamp,
Thesis University of Twente (The Netherlands), 1995.
[16] M. Nigam, C. M. Seong, Y. Qian, M. A. Blaskovich, A. D. Hamilton, S. M.
Sebti, J. Biol. Chem. 1993,268,20695-20698.
[17] a) M. L. Smythe, M. van Itzstein, J. Am. Chem. SOC.1994,116.2725-2733;
b) A. C. Bach, C. J. Eyerman, J. D. Gross, M. J. Bower, R. L. Harlow, F! C.
Weber, W. E DeGrado, ibid. 1994,116,3207-3219.
Angew. Chem. Int. Ed. Engl. 199736, No. 23
BaSn,: A Superconductorat the Border of Zintl
Phases and Intermetallic Compounds.
Real-Space Analysis of Band Structures**
Thomas F. Fassler* and Christian Kronseder
In memory of Jeremy Burdett
Superconducting intermetallic compounds that are rich in
main group elements, especially the alloys that crystallize in
the Cu3Austructure type, have been a focus of research owing
to the interesting dependence of their critical temperature T,
on their valence electron countsjl] In the course of our
investigations on phases rich in germanium, tin, and lead, we
discovered superconducting compounds in the latter two
systems. We report here the pure-phase synthesis, magnetic
properties, and electronic structure of BaSn,, which crystallizes in the Ni3Sn structure type (hexagonal variant of Cu3Au).
Results of band-structure calculations on BaSn3 that predict
anisotropic conductivity and show very flat bands at the Fermi
edge EF were the impetus for our magnetic studies. The
coincident appearance of quasi-molecular states (localization)-and therefore a high density of states at E-with
steep, disperse bands crossing the Fermi level (delocalization)
may be viewed as a “fingerprint” in the search for new
The hypothesis that pairing localization of
conducting electrons occurs in super~onductors[~1
was successfully tested with several model systems, for example
(RE),X,C, (RE =rare earth metal, X =halogen) and La,Br,(CBC)3.12-4]
Note that even in the case of the superconducting
cuprates, the maximum transition temperature corresponds to
a high density of states at EF(van Hove singularity) .I5]
[*I
[**I
Dr. T. F. Fassler, C. Kronseder
Laboratorium fur Anorganische Chemie der
Eidgenossischen Technischen Hochschule
Universitatstrasse 6, CH-8092Zurich (Switzerland)
Fax: Int. code+(1)632-1149
e-mail: faessler@inorg.chem.ethz.ch
This work was supported by the Eidgenossischen Technischen Hochschule
Zurich. We thank D. van Arx and M. Spahr for performing the magneticsusceptibility measurements and Prof. R. Nesper for friendly support.
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BaSn, is a model superconducting compound, as it is the
first material discovered at the border of intermetallic
compounds and Zintl phases that exhibits these characteristics. In this work the electronic properties of BaSn, are
rationalized using calculations of band structure and the
electron localization function (ELF) A real-space analysis
of the band structure is accomplished with the aid of
direction-dependent partial electron density (PED) .L71 Such
a depiction of the crystal-orbital densities allows an analysis of
the band structure in real space that is unachievable with
other methods,["] and provides intuitive pictures for solidstate compounds that are similar to those provided by the
popular orbital concept for molecular systems.
BaSn, was synthesized as a pure phase by the stoichiometric
combination of the elements.[1b16]BaSn, crystallizes in the
primitive hexagonal Ni,Sn structure type (space group
P6,/mmc, a = b = 7.253(2), c = 5.496(2) A) with the tin atoms
at the nickel positions.[16] This structure type, which is
common for intermetallic compounds, occurs by a 1:3
distribution of the Sn and Ba atoms within hexagonal layers
(small and large circles in Figure 1a) that stack in an ABAB
sequence (Figures 1a and 1b). The distortion within the
layers of Sn atoms (Wyckoff site: (6h)x, 2x, 1/4: x = 0.8594)
with respect to the corresponding sites in the anstotype
structure ( x = 0.8333) leads to shorter contacts between Sn
atoms within a layer (Sn-Sn 3.058
and longer distances
between Sn and Ba centers (Sn-Ba 3.663 A). The compression along the c axis (cla = 0.758) manifests itself in short
interla er distances between Ba and Sn atoms (Ba-Sn
3.641 ); however, the Sn-Sn interlayer contacts (3.266 A)
are longer than the intralayer contacts.
The topology of the tin framework (Figure lg) emphasizes
the relationship to the hexagonal perovskite structure
(BaNiO,) and a description according to the concept of Zintl,
Klemm, and Busmann. BaSn, may be described as a defect
structure in which tin atoms are at the oxygen positions, and
the centers of the octahedra are unoccupied (BaUSn,). The
one-dimensional chains running along the c axis are constructed from elongated face-sharing octahedra and may be
described as 2 [Sni-1, a formula that invokes the transfer of
Ba valence electrons to the Sn polyanion. If only the shortest
Sn-Sn contacts are considered to be bonding, the tin atoms
may be viewed as forming triangular Sn g- ions (Scheme 1).[I7]
Such units are isoelectronic with aromatic 2nelectron cyclo-
x
5,6
++
+
4
Scheme 1. MO scheme of a Sn:- Zintl anion.
Figure 1. Structure and graphical analysis of the electronic structure of BaSn,.
a) A hexagonal layer with a 1:3 distribution of the atoms, b) AB stacking of two
such layers ( z = 114 and 3/4), c) distortion from ideal hexagonal packing, d) the
ELF in a plane through the atoms of an isolated one-dimensional 2 [Snd-] chain
of octahedra, e) the ELF in BaSn, (same plane as in d)) ,f) a larger cross-section
from the BaSn, structure, g) 3D representation of the face-sharing tin octahedra.
ELF =0.80 (yellow); the PED (red) isosurfaces of the band sections 1 (right) and
2 (left) have been added. The PED was calculated for the thick part of the flat
bands near the Fermi level (between M and r) in Figure 3.
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propanyl cations (C3R2 ). Therefore, the chains of facesharing octahedra may also be viewed as a stacking of
triangular units with strong interactions between the component JL systems.
Measurements of the magnetic suseptibility at low field and
at various temperatures showed the typical curves for
diamagnetic shielding and the Meissner effect (T, = 4.3 K,
Figure 2) .[**I The transition temperature of BaSn, can be
clearly differentiated from that of tin (3.7 K), as proven by
similar measurements on elemental Sn and a simultaneous
measuxement of a mixture of Sn and BaSn, (inset of Figure 2 ) .
The density of states and a section of the band structure
obtained from LMTO calculations are shown in Figure 3.["]
The band structure of BaSn, (Figure3a) features a large
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I -
0.0
2
4
-
8
6
TIK
Figure 2. Diamagnetic shielding (0)and Meissner effect ( 0 ) in BaSn, as a
function of temperature. The inset shows the results of a similar measurement on
elemental tin.
d)
/--
r
K M
r
A
L
H
A
Figure 3 Band structure and density of states of Ba S q The contnbutions of the
b) px,p,, and c) pz orbitals the tin atom are plotted The Fermi level is taken as
the point of zero energy The Brillouin zone is shown in d)
dispersion in the bands along the symmetry lines perpendicular to the distorted hexagonal layers; the bands parallel to
the layers are comparatively flat. An anisotropic metallic
conductivity is apparent in BaSn, with the special situation
that flat bands and a degeneracy (in the zone center r) appear
at the Fermi level. This Ieads to an increased density of states
at EF (Figure 3a, right). A “fat-band’’ analysis, in which the
thickness of the band represents the corresponding orbital
contribution, shows that the flat portions of the bands in the
vicinity of EForiginate from px and py orbitals of the Sn atom
(Figure 3b). Conversely, in the sections featuring steep bands,
such as from r to A (i. e. parallel to the octahedral chains,
z direction), increased mixing with the pz orbital is observed
Angew. Chem. Int. Ed. Engl. 199x36, No. 23
(Figure 3c). From real-space PED plots, sections 1 and 2 of
the band structure depicted in Figure 3a may be assigned to
the molecular orbitals of the triangular Sn, unit shown in
Scheme 1.
The result of the graphical analysis~zo]of an extendedHiickel
which enables a qualitative analysis of
the LMTO band structure, is shown in Figure lg. Here the
PED isosurface of the flat band from section 1 (along the
M-I? line) is shown on the right octahedral chain, and that of
section 2 on the
The analysis shows that the bands
below EFarise from the lone electron pairs (MO 7, Scheme 1)
with mixing of the “tangential orbitals” of the tin triangle (see
MOs5 and 6); above ER only the tangential orbitals are
apparent. It is remarkable that the “MO-like” pictures in
Figure l g were obtained from calculations of band structures
with the underlying symmetry of the kpoints in reciprocal
space, and not calculated from points of higher symmetry
(e. g. only at r),which is the usual method for depicting the
wave
This enables consideration of anisotropic
properties in real space.
To pinpoint the interaction between the free electron pairs
at the Fermi
ELF analyses were
Figure I d shows the ELF of a layer parallel to the xy plane
through three tin atoms of an isolated i [ S n % - ]octahedral
chain. One can clearly discern three small bright areas, which
are consistent with the Sn-Sn bond lengths that are about
0.2 A shorter; this supports the formulation as a Snz- Zintl
Three outer areas with half-moon shapes correspond to
the free electron lone pairs of the tin atoms. The same crosssection from the full BaSn, structure is shown in Figure le,
and a magnification of a unit in Figure If. The lone pairs are
clearly flattened owing to interactions with neighboring
electron pairs. This correlates with the strong lowering of
the bands at the Fermi level (section 2) that is observed in
calculations for which a larger separation of the octahedral
chains is effected by increasing the unit-cell parameters but
retaining the same Sn-Sn bond lengths. The “compression” of
the lone pairs in BaSn, may also be seen in three-dimensional
ELF plots (yellow isosurface in Figure 1s).
The results obtained with PED and ELF demonstrate that
the interaction of the lone pairs parallel to the layers are
responsible for the band dispersion and for the elevation of
certain bands at the Fermi level. The following qualitative
picture of the electron - phonon interactions may be deduced:
Lattice vibrations have a strong effect on the interactions
between the lone pairs. Raising the states of the free electron
pairs above the Fermi level results in electron transfer to the
conduction band (and vice versa), that is, transitions between
localized and conducting electrons may be dependent on
lattice vibrations.
BaSn, may certainly be viewed as a borderline Zintl phase.
Strong “interanionic” interactions between the n systems of
the anionic units and weak interactions between the lone pairs
must be considered. Similar situations were also found in
other compounds. Interactions leading to 1D metallic properties were seen between the n systems of the central Si6rings in
the stacked planar [Si,,]20.5- anions of Ca,Mg,.2,Si4.[251Parabolic arrangements of lone pairs were observed in the
intermetallic compound RhBi,1261and in the caverns of the
lead substructure of K5Pbz4-[81Both RhBi,[271 and K,Pb2J8]
exhibit superconductivity. Superconductivity is also observed
in the silicides (Na,Ba),Si46.[281The silicon atoms are tetravalent in the basic clathrate(1) structure type
where A =
K, Na.[291Assigning the composition of some compounds of
this structure type as A4EZ20,(A =alkali metal, E = Si, Ge)
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requires that one of the sites in the 3D network of E atoms is
vacant.[30]Since sites adjacent to the empty position contain
three-bonded atoms (that are isosteric to the Group 15
elements), these compounds may also be described as Zintl
phases with interacting electron pairs. A strong interdependent influence of neighboring electron pairs of three-bonded
atoms can also be found in the Zintl phase K,Sn,Bi,.[s.311
These observations can be interpreted as evidence for a
correlation between the presence of localized states in the
form of lone pairs in metallic conductors and superconductivity. Further examples and analyses may lend support to this
theory. For BaSn,, which resides on the border of intermetallic
compounds, it appears that a balanced interplay of interactions between localized and delocalized structural components leads to the development of specific metallic properties.
Received: June 19,1997 [Z10569IE]
German version: Angew. Chem. 1997,109,2800-2803
Keywords: electron localization function
phases superconductors Zintl phases
-
-~
-
-
intermetallic
~
E. E. Havinga, Phys. Len. A 1968, 28, 350; E. E. Havinga, H. Damsma,
M. H. van Maaren, Phys. Chem. Solids 1970,31,2653.
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108,1805; Angew. Chem. Inf. Ed. Engl. 1996,35,1685.
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Felser, R. K. Kremer, H. Mattausch, 2. Anorg. Allg. Chem. 1996,622, 123.
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Fassler, Angew. Chem. 1997,109,1892;Angew. Chem. Int. Ed. Engl. 1997,36,
1808.
The PED was calculated by summing the square of the wave function at
respective kpoints lying inside a selected energy window. This can be
accomplished using a kpoint set standardized over the first Brillouin
zone [9] or, as done here, by choosing k points along selected symmetry
lines. Using the former method to calculate the PED over an energy window
either above or below EFleads to a density of states around the Fenni level
that may be used to simulate tunneling electron microscope images [8-lo].
T. F. Fassler, Habilitation thesis, ETH Ziirich, 1997.
T.F. Fassler, U. Haussermann, R. Nesper, Chem. Eur. J. 1995, I, 625.
M.-H. Whangbo, S. N. Magonov, Adv. Mafer. 1994,6,355.
See, for example, the fat band analyses in ref. [12] or the use of localized
band orbitals with the aid of Wannier functions [13].
Program TB-LMTO-ASA: M. van Schilfgarde, T. A. Paxton, 0. Jepsen,
0. K. Andersen, G. Krier, Max-Planck-Institut fur Festkorperforschung,
Stuttgart, 1994; U. Barth. L. Hedin. J. Phvs. Chem. 1972.5.1629; 0.Jeusen,
0. KrAndersen, 2. Phys. B 1995, 97,35.[13] K. A. Yee, T. Hughbanks, Inorg. Chem. 1991,30,2321; ibid. 1992,31,1620;
Y. Tian, T. Hughbanks, ibrd. 1993,32,400.
[14] Stoichiometric amounts of barium (0.538 g, surface impurities were
removed immediately before loading) and tin powder (1.512 g) were heated
in niobium ampoules at 150 degress per hour to 720°C. The ampoules were
held at 720°C for 48 h and then cooled at 50 degrees per hour to 610°C.
They where then held at this temperature for 16 d. BaSn, was isolated as a
microcrystalline, shiny silver powder. Resistivity measurements on pressed
pellets show a linear increase with temperature in the region from - 200 to
25°C. A stable phase with this composition was identified in 1930 from
thermal studies on the BdSn system [15], and a single crystal isolated from
investigations in a ternary system (BdAUSn) was structurally characterized [16]. We obtained single crystals of BaSn, in the investigation of the
ternary WBdSn system.
[15] K. W.Ray, R. G. Thompson, Met Alloys 1930,l. 314.
[16] R. Kroner, Dissertation, Universitat Stuttgart, 1989.
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0 WILEY-VCH Verlag GrnbH, D-69451 Weinheim, 1997
Typical Sn-Sn bond lengths: 2.78 A in (Ph,Sn),: D. H. Olson, R. E. Rundle,
Inorg. Chem. 1%3, 2, 1310; 2.77 A in Ph,Sn,: H. Preut, H.-J. Haupt, F.
Huber, Z. Anorg. Alig. Chem. 1973,396.81; 2.85 - 2.87 8,in (Ar,Sn), (Ar=
2,6-diethylphenyl): S. Masamune, L. R. Sita, J. A m Chem. SOC. 1983, 105,
630; 2.98 8,in K4Sn4:I. F. Hewaidy, E. Busmann, W. Klemm, 2.Anorg. Allg.
Chem. 1964,328, 283; further examples: D. E. Goldberg, P.B. Hitchcock,
M. F. Lappert, K. M. Thomas, A. J. Thorne, T. Fjeldberg, A. Haarland,
B. E. R. Schilling,J. Chem. SOC.Dalton Tram. 1986,2387.
The magnetic susceptibility was measured with a SQUID magnetometer
(MPMS 5, Quantum Design). The samples were cooled in the absence of a
magnetic field. After introducing a field of 10 G, the sample was warmed
(“shielding”) and then cooled (“Meissner”). Suprasil quartz capillaries
(5 mm diameter) served as sample holders. The shoulder at 3.7 Kin the plot
in Figure 2 can be assigned to trace amounts of elemental tin. As no lines
characteristic of tin can be seen in an X-ray powder pattern, an impurity
level of less than 5 % tin can be estimated.
Calculations were performed with the program TB-LMTO-ASA[lZlwithout
the addition of interstitial spheres with s-, p-, and (“down-folded”) d-partial
waves for Ba and Sn.
The analyses were carried out with the program MEHMACC (U.
Hausermann, S. Wengert, R. Nesper, T. F. Fassler, ETH Zurich, 1993); the
pictures were created with the program COLTURE for the isosurfaces (P.
Hofmann, R. Nesper, ETH Ziirich, 1993) and GRAPA for the cross-sections
(J. Flad, F.-X. Fraschio, B. Miehlich, Institut fur Theoretische Chemie
der Universitat, Stuttgart, 1989). The programs are available under
mehmacc@inorg.chem.ethz.ch. MEHMACC is based on the QCPE
extended-Huckel program EHMACC [21].
Program EHMACC: M.-H. Whangbo, M. Evain, T. Hughbanks, M. Kertes,
S. Wijeyesekera, C. Wilker, C. Zheng, R. Hoffmann, 1990; relevant
parameters (H,,, 5): Sn 5 s - 16.16 eV, 2.12, 6 p -8.32 eV, 1.82; Ba 6s
-4.76 eV, 1.263, 6 p -2.64, 1.263.
[22] In the extended-Hiickel band structures of band section 1, no crossing of the
next-highest band is observed between M and r.
1231 R. Hoffmann, Solids and Surfaces. A Chemist’s View of Bonding on
Extended Surfaces, VCH, Weinheim, 1988.
[24] The possibility of nonbonding electron pairs participating in the pairing
localization of electrons in superconductors is also mentioned in ref. [4a].
[25] A. Currao, S.Wengert, R. Nesper, J. Curda, H. Hillebrecht, Z. Anorg. Alig.
Chem. 1996,622,501.
[26] Y. Grin, U. Wedig, H. G. von Schnering, Angew. Chem. 1995, 107, 1318;
Angew. Chem. Inf. Ed. Engl. 1995,34, 1204.
[27] N. E. Alekseevskii, G. S. Zhandov, N. N. Zhuravlev, Zh. E h p . Teor. Fiz.
1955,28, 237.
[28] H. Kawaji, H. Hone, S. Yamanaka, M. Ishikawa, Phys Rev. Lett 1995.8,
1427.
1291 I. S. Kaspar, P. Hagenmiiller, M. Pouchard, C. Cros, Science 1%5, 1713; J.
Gallmeier, H. Schafer, A. Weiss, 2. Nafurforsch. B 1%7,22, 1080.
[30] H. G. von Schnering, Nova Acfa Leopold. 1985,59,168.
[31] T. F. Fassler, C. Kronseder, Z. Anorg. Allg. Chem., in press; T. F. Fassler,
ibid., in press.
Carbohydrates Coordinated to Platinum(1v)
through Hydroxyl Groups: A New Class of
Platinum Complexes with Bioactive Ligands**
Dirk Steinborn,” Henrik Junicke, and Clemens Bruhn
The discovery of the carcinostatic effect of cis[PtCI,(NH,),] by Rosenberg et al.[*] and investigations on
the mechanism of action of platinum compounds in chemotherapy[*] promoted the developement of the coordination
chemistry of platinum@) with biologically important lig a n d ~ . [ ~Nowadays
-~]
attention is focused on platinum(1v)
complexes with bioactive ligands, because of the lower
toxicity of platinum(1v) and the possibility of oral administration of some potent platinum(1v) compounds as well as the
[*] Prof. Dr. D. Steinborn, DipLChem. H. Junicke, Dr. C. Bruhn
Institut fur Anorganische Chemie der Universitat Halle-Wittenberg
Kurt-Mothes-Strasse 2, D-06120 Halle (Germany)
Fax: Int. code + (345) 552-7028
e-mail: steinborn@chemie.uni-halle.de
[**I This work was promoted by the Deutsche Forschungsgemeinschaft and the
Fonds der Chemischen Industrie, and supported by gifts of chemicals from
Degussa and Merck.
0570-083319713623-2686$17.50+.50/0
Angew. Chem. Inf. Ed. Engi. 1997,36, No. 23
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